Magnesium and/or calcium based particles or granules are widely used in a number of applications, including in cements, fertilizers, and sorbents.
Most commonly used cements are formed with calcium as the elemental basis for the cementitious component, generally in the form of lime, CaO. These cement formulations include lime, Portland cement, and Portland type blended cements such as Portland-slag cements and Portland limestone cements. Portland cement is produced from ground clinker, and clinker is a sintered mixture of ground lime and argiciilaceous components (SiO2, Al2O3, and Fe2O3) from clay, shale, marl, tuff, ash phyllite and slate, mixed in proportions to provide a network with compositions 3CaO.SiO2 (50-70%), 2CaO.SiO2 (15-30%), 3CaO.Al2O3 (5-10%) and 4CaO.Al2O3.Fe2O3 (5-15%).
The hydration of such calcium based cements produce an excess of Portlandite, Ca(OH)2. Portlandite has a high solubility, and thus has a mobility and reactivity that is known to result in poor quality cements, unless otherwise suppressed. For good cement, the excess Portlandite is reacted, most usually with activated Pozzolans. Upon setting, the calcium is then locked into low solubility compounds, and strong cements are produced. Many formulations of good calcium based cements are now understood to be those that minimise Portlandite during hydration. There are other requirements for good cement—for example, small volume changes on setting, uniform setting rates of the cementitious components, and these are achieved through the choices of binders, fillers, accelerators and the like. However, an essential challenge of good calcium based cement formulations is to match the activity of the Pozzolans so as to consume the Portlandite. This restriction limits many formulations of calcium based cements.
On the other hand, the equivalent magnesium based cements, where they can be made, do not have the same fundamental problem as calcium based cements because Brucite, Mg(OH)2 is insoluble, and does not migrate or readily react. However, many of the earlier magnesia based cements, such as the Sorrel cements, have poor resistance to water degradation because they use magnesium oxychioride and magnesium oxysulphate, as the cementitious component, and the chloride and sulphate ions are responsible for the poor properties. These materials were used in cements because reactive (caustic) magnesia MgO was then not readily available. The calcination of magnesium carbonate in conventional kilns yielded dead-burned magnesia in which the reactivity had been reduced by sintering, and had a poor reactivity in cements, similar to the mineral Periclase. Periclase hydrates slowly, and is not a desirable component for cements. It was the difficulty of obtaining reactive magnesia on an industrial scale that lead to the development of the Sorrel cements.
Magnesia based cements generally require additional additives, such as sodium silicate because the hydrate itself does not have sufficient anions to form a gel that sets. Thus rapid setting magnesia based cements are used in many applications (repairs etc) and have complex formulations.
Reactive magnesia, sometimes called caustic magnesia, can now be produced by careful calcination, and is understood to produce excellent cements. The lower solubility, by five orders of magnitude, of Brucite compared to Portlandite means that there can be a greater diversity of cements possible when magnesium is used instead of calcium because the constraint of solubility is lifted. That is, formulations of these cements do not require that the Brucite is consumed. Magnesium based cements have other properties that make them desirable, for example they are lightweight and with a low thermal conductivity, with these being properties that are largely a property of the magnesium ion in such materials.
While magnesium based cements may have desirable properties, the deposits of the mineral magnesite MgCO3 are limited, and the cost of extraction of magnesia from brine is expensive. That is, magnesium based cements cannot generally compete with calcium based cements on price, except for niche applications
There exist mixed calcium and magnesium based cements. For example, the excess Portlandite in Portland cements can be consumed by using reactive magnesia, which forms insoluble Brucite Mg(OH)2. The amount of magnesia required to achieve this formation can be relatively small, and these cements have many desirable properties.
Despite the many common properties of calcium and magnesium based cements and their mixtures, there are very few cement formulations that are produced from calcined dolomite. Dolomite is a plentiful mineral, with a crystalline structure based on units of MgCO3.CaCO3. The structure consists of layers of strictly alternating CaO6 and MgO6 octahedra with carbonate layers sandwiched between them. That is, the calcium and magnesium ions are in close proximity in the crystal structure. Dolomite can be completely calcined in conventional kilns to give a material, Dolime (MgO+CaO), which is comprised of microcrystallites of magnesia (MgO) and lime (CaO). The phase separation of MgO and CaO is a consequence of thermodynamics, namely that crystals of MgO and CaO have a lower free energy than a crystal MgO.CaO in which the magnesium and calcium are together in the unit cell of the crystal. Cements based on Dolime are not satisfactory because the magnesia microcrystals sinter to Periclase during calcination, which is unreactive, and hydrates more slowly than the lime.
Cements based on partially calcined dolomite are described by Rechichi (U.S. Pat. No. 6,200,381) in which dolomite is partially calcined to a mixture of calcium carbonate and partially calcined magnesium oxide. It is understood that such partial calcination produces a mixture of MgO and CaCO3 microcrystals (J. E. Readman and R. Biom Phys. Chem. Chem. Phys 2005, 7, 1214-1219). Rechichi demonstrated the equivalent of making the same cement from a mixture of separately partially calcined MgO and limestone granules. These mixture-based cements, often with additives such as inorganic salts (acids, magnesium sulphate, and aluminium sulphate) have many desirable properties and applications, which include the ability to use organic materials and many other materials as binders, and the use of sea water in the setting process. In the cements based on these mixtures, however, the CaCO3 acts largely as filler and the benefits of using dolomite feedstock are not clear. The cements are not substantially different from those made from partially calcined MgO.
It has been demonstrated in the laboratory that experimental conditions can be found in which the dolomite decomposes at 640-700° C. into a mixture of a solid solution of MgO and CaCO3 and MgO and CaCO3 microcrystals, with considerable sintering of MgO. (D. T. Beruto, R. Vecchiattini and M. Giordani, Thermochim. Acta 2003, 404, 25 (2003)). In later work they were able to produce this material under high pressure of CO2 in a Knudsen cell. (D. T. Beruto, R. Vecchiattini and M. Giordani, Thermochim. Acta 405, 183 (2003)). The solid solution was identified as an intermediate in the high temperature thermal decomposition of dolomite to MgO and CaCO3 microcrystals, and no conditions were identified to produce the pure solid solution. Under the experimental conditions, the sintering of the materials would give low reactivity, There has been no means of manufacture described for this material on an industrial scale.
All the cements above—calcium, magnesium and mixed calcium/magnesium—react with carbon dioxide (CO2) in a carbonation process that is the reverse of calcination. The carbonate ions produced by carbonation have stronger bonds that than the hydroxides, so that the cement strength will, in certain circumstances, increase over time as carbonation takes place. However, carbonation of cement develops from the exposed surface to give an inhomogeneous material, and volume changes can induce the development of stresses. This can lead to deterioration of the cement over time with carbonation. Cements can be formulated so that the volume change is small, or the carbonation is slow. However, this requirement also limits cement formulations
The impact on greenhouse emissions from cement manufacture is significant, with over 3% of anthropogenic emissions coming from this source. The release of CO2 during calcination and its slow recapture by carbonation means that the CO2 released impacts on global warming. Only a small fraction of cement re-carbonates quickly because the majority of cement is too far from an exposed surface. Only about 40% of the CO2 liberated during calcination can be re-carbonated in optimal circumstances. For example for Portland cement, the high temperatures used to make cement clinker lead to ˜100% calcination.
For cements produced from lime slurry, the use of partially calcined lime granules in the slurry leads to a phase separation into granules of Portlandite Ca(OH)2 and calcite CaCO3, and the calcite is incorporated into the cement as filler. There is no net advantage in greenhouse emissions because the calcite replaces other fillers, and there is no emissions savings. The high solubility of Portlandite is responsible for the phase separation of the partially calcined lime during hydration. It is noted that careful control of calcination of limestone can produce a material in which 50% partial carbonation is achieved, namely CaO.CaCO3. This material phase separates into Portlandite and calcite as described above, when hydrated.
For magnesia based cements, the partial calcination of magnesite granules can be achieved, but this is inhomogeneous as the calcination proceeds from the granule surface inwards. The low solubility of Brucite and magnesite is such that this separation is retained in the hydration process, and the cement is similar to magnesia based cement that uses magnesite as filler. There is no net advantage in greenhouse emissions because the magnesite replaces other fillers, and there is no emissions savings. The low solubility of Brucite is responsible for the poor mixing during hydration.
In summary, there has been no practical cement manufacturing process for calcium or magnesium based cements that can take the advantages of the greater strength of a carbonate matrix and a reduction in greenhouse gas emissions. Cements generally required additional materials, such as Pozzolans and soluble silicates, to provide anions required for a strong matrix.
Ideally, on a volume basis, the energy used in calcination and the heat given off in forming cement should be minimised. The release of heat during formation of slurry and/or in mixing and setting is a measure of the energy imbalance in the formation of cements. A reduction of the energy demand would be beneficial with regard to cost of production. If the hydration and setting is done in a single step, as in Portland cement, the temperature rise can cause strains in the cement when it cools. To minimise the energy inputs of calcinations, an existing technique is to reduce the amount of cementitious material by fillers. The use of partially calcined materials, does not work for either lime or magnesia, as mentioned above. Carbonation of calcium and magnesium based cements is a further indication that the thermodynamics of these cement formulations is not optimised. That is, many cement formulations, after setting, have too high a free energy and are chemically unstable.
In relation to fertilisers, magnesium deficiency of soils is a widespread problem, and commonly occurs in acidic soils. The magnesium deficiency is often dealt with by using ground dolomite, but it is understood that the beneficial effects do not appear because of the low solubility of the dolomite. In acid soils, the addition of a basic material, such as lime, has the beneficial effects of increasing the pH and unlocking essential nutrients from the soil. It is understood the use of magnesia, or hydrated magnesia, can provide both the magnesium and the acid neutralization. However, magnesia is often produced as Periclase, which is sintered and unreactive. For example, Periclase only slowly hydrates to Brucite. Reactive magnesia is not common, and is believed to be too expensive. Heating dolomite in conventional kilns also leads to an unreactive Periclase.
In relation to sorbents, lime and hydrated lime are widely used as sorbents in industrial applications, and use calcium as the active element. There are a number of applications in which magnesium has more desirable properties—for example, in some processes that require a reversible process, the binding of compounds to calcium is too strong, and the temperature of the reverse reaction (eg calcination) is considered to be too high. Generally, magnesium has a lower binding energy than calcium, leading to a lower temperature to reverse the binding. Even where magnesium is preferred, the cost of magnesia may be too high.
In relation to sorbents, particularly for sulphur and carbon capture, the use of calcined limestone as a sorbent has been well studied. The reactivity of sorbents scales with the surface area and pore sizes. Small pore sizes, namely micropores <2 nm tend to clog with sorbate, and the rapid sorption efficiency plateaus. There is a longer process in which the sorbate diffuses through the granules, but this is not generally used commercially. It is understood that in materials produced by conventional calcination, the surface area and pore sizes are reduced by sintering and the reactivity is reduced. It is now also understood (J. C. Albandades and D. Alvarez, Energy and Fuels, 2003, 17, 308-315) that it is desirable to produce sorbents that have not only a high surface area, but also a high mesoporous volume, with mesopores being about 5-20 nm diameter. Mesopores are less prone to dogging, and these materials have a high degree of fast sorption. Fan and Gupta (U.S. Pat. No. 5,779,464) describe the fabrication of a high mesoporous “super sorbent” from limestone using the process of calcining limestone to lime, forming a hydrate and dissolving the calcium, and precipitating small calcium carbonate particles and calcining these particles. For many price sensitive industrial applications of sorbents, such as carbon capture, the process steps described by Fan and Gupta will add a cost that may make their “super sorbent” too expensive.
There are advantages in developing a magnesium based sorbent derived from magesite or dolomite because the energy and temperature requirements for the calcination re-processing are significantly reduced, without loss of the degree of binding required. In many applications in which reversible sorption is required, the magnesium material is preferred over calcium because the enthalpy of desorption is lower with the magnesium reaction than the calcium reaction. Thus, the temperature of decomposition of CaCO3 to CaO (calcination) is about 895° C. whereas that for MgCO3 to MgO is about 395° C. Further, the enthalpy of the reaction with the magnesium is smaller than that of the calcium. In many industrial processes, these are significant differences. However, many experiments on the use of MgO as a sorbent show a limited degree of adsorption because the morphology of the MgO materials. Firstly, MgO produced by calcination is generally sintered, so that the surface area is not large and the sorption capacity is low. And secondly, the micropores in the surface of MgO (ie <10 nm) readily clog with the adsorbed material such that penetration of the sorbate into the granule is restricted. For example, magnesium oxide slurry is limited in its sorption properties by pore clogging (D. P. Butt, et al J. Am.Ceram. Soc. 1996, 79, 1892-1898 (1996)). As a consequence, the initial rapid adsorption onto the available surface saturates at a low level because of the low surface area, and a slower process takes place in which the sorbate has to diffuse through the material. Only the initial rapid reaction is of industrial interest. As a consequence, CaO is generally preferred as a sorbent over MgO.
It has been established that calcined dolomite is a sorbent for many materials, such as carbon dioxide and sulphur dioxide, in many applications in which lime and or magnesia having a high surface area may be generally used. Dolime is produced by the slow calcination of dolomite at high temperatures (>900° C.) is known to produce microcrystallites of MgO+CaO, and the sorption properties of this material are dominated by the CaO microcrystals because the MgO microcrystals are strongly sintered and unreactive. This material may have a similar reversible sorption properties as lime (J. Readman and R. Blom, Phys Chem Chem Phys, 2005, 7 1214-1219). The slow partial calcination of dolomite at temperatures below 700° C. yields MgO+CaCO3 microcrystals, and the sorption properties of this material are similar to that of MgO described above, and is generally a poor sorbent. The CaCO3 plays little role.
Thus there currently exists no magnesium based “super-sorbent”, based on either calcined magnesite or dolomite that can take advantage of the preference for magnesium based sorption compared to calcium based sorption.
A need therefore exists to provide a material compound and a method of fabricating the same which, seek to address at least one of the above-mentioned problems.